Bitumen (colloquially known as “tar” due to its similar appearance, odor, and color) is a thick, sticky form of crude oil, so heavy and viscous (thick) that it will not flow unless heated or diluted with lighter hydrocarbons. Bituminous sands—colloquially known as oil sands (or tar sands) contain naturally occurring mixtures of sand, clay, water, and bitumen and are found in extremely large quantities in Canada and Venezuela.
Conventional crude oil is normally extracted from the ground by drilling oil wells into a petroleum reservoir, and allowing oil to flow into the wells under natural reservoir pressures. Artificial lift techniques, such as water flooding and gas injection, are usually required to maintain production as reservoir pressure drops toward the end of a field's life, but initial production proceeds under normal reservoir pressures and temperatures.
Oil sands are very different however. Because extra-heavy oil and bitumen flow very slowly, if at all, toward producing wells under normal reservoir conditions, oil sands must be extracted by strip mining or the oil made to flow into wells by in situ techniques that reduce the viscosity by injecting steam, solvents, gases or other forms of energy into the sands to heat or otherwise reduce the viscosity of the heavy oil. These processes can use more water and require larger amounts of energy than conventional oil extraction, and thus heavy oils cost more to produce than conventional oils.
The use of steam injection to recover heavy oil has been in use in the oil fields of California since the 1950s. In Cyclic Steam Stimulation (“CSS”) or “huff-and-puff” the well is put through cycles of steam injection, soak, and oil production. First, steam is injected into a well at a temperature of 300 to 340 degrees Celsius for a period of weeks to months. The well is then allowed to sit for days to weeks to allow heat to soak into the formation. Later, the hot oil is pumped out of the well, again for a period of weeks or months. Once the production rate falls off, the well is put through another cycle of injection, soak and production. This process is repeated until the cost of injecting steam becomes higher than the money made from producing the oil. The CSS method has the advantage that recovery factors are around 20 to 25% and the disadvantage that the cost to inject steam is high, and it is often not cost effective to produce heavy oil this way.
Steam Assisted Gravity Drainage (SAGD) is another enhanced oil recovery technology that was developed in the 1980s and fortuitously coincided with improvements in directional drilling technology that made it quick and inexpensive to do by the mid 1990s. In the SAGD process, at least two parallel horizontal oil wells are drilled in the formation, one about 4 to 6 meters above the other. Steam is injected into the upper well, possibly mixed with solvents, and the lower one collects the heated crude oil or bitumen that flows out of the formation, along with any water from the condensation of injected steam.
The basis of the SAGD process is that the injected steam forms a “steam chamber” that grows vertically and horizontally in the formation. The heat from the steam reduces the viscosity of the heavy crude oil or bitumen, which allows it to gravity drain into the lower wellbore. The steam and gases rise because of their low density compared to the heavy crude oil below, ensuring that steam is not produced at the lower production well.
The gases released, which include methane, carbon dioxide, and usually some hydrogen sulfide, tend to rise in the steam chamber, filling the void space left by the oil and, to a certain extent, forming an insulating heat blanket above the steam. The condensed water and crude oil or bitumen gravity drains to the lower production well and is recovered to the surface by pumps, such as progressive cavity pumps, that work well for moving high-viscosity fluids with suspended solids.
Although SAGD techniques have been very successful, one factor that can limit the economic production of the viscous oil using SAGD is the heterogeneous nature of the reservoir. The applicability of SAGD is often limited by impermeable layers (such as shale and mudstone) that act as barriers to vertical flow. The impermeable layers effectively compartmentalize the reservoir into thin sub-reservoirs, less than 15 meters in length at its minimum. These thin layers cannot be economically developed with gravity drainage processes because of the thickness requirement for cost effective production.
Thus, what is needed in the art are methods of improving the cost effectiveness of recovering heavy oils, even in heterogeneous reservoirs that are vertically compartmentalized.